Dry mixes were first introduced by General Mills during the late 1940's. They originated as a commercial mixture of dry ingredients that were intended to simplify and streamline the baking process. Prepared dry mixes for layer cakes are generally made by combining sugar, flour, shortening, leavening and other ingredients including perhaps milk solids, egg solids, flavoring and coloring to form a free-flowing granular mixture. To prepare cake batters from these dry mixes for baking, liquid materials such as water, liquid shortening, milk and eggs are added and the combination is then beaten both to form a homogeneous mixture and to incorporate air. The resulting batter is then baked for a pre-determined amount of time at a specific temperature in a conventional oven.
In response to consumer demands for convenience, a wide variety of pre-made food items have been designed for heating in a microwave oven. Popular examples of these items include lasagna, cheese macaroni dishes, and vegetable casseroles. Batters from consumer culinary mixes designed for conventional oven baking can also be microwave baked. However, microwave baking presents unique challenges that are not encountered with conventional ovens. As a result, the application of microwave technology to dry mix desserts has been somewhat slower to develop.
When a conventional dry mix designed for oven baking is prepared into a batter and baked in a microwave, the finished layer cake texture can be fine textured and spongy. Often, specific gravities are much higher and pronounced variations from edge to center are observed. Moreover, the center of the microwave-prepared cake can be soggy and dense, while the edges are often overly dry. This is especially true in larger sized containers. Center point depressions can also occur.
Many of the deficiencies in finished layer cake quality can be attributed to the fundamental differences in the baking mechanisms between conventional ovens and microwave ovens. Conventional ovens rely on three methods for transferring heat—conduction, convection, and radiance. Conduction transfers heat through direct contact of a hot surface to a cold one. Convection transfers heat by moving a hot material such as air. Radiant heat transfer works by transmitting heat energy in a radiant form, similar to the way light is transmitted.
In contrast, microwave ovens do not transfer heat to a material. Rather, the material is induced to heat itself as the microwave oven generates a continually changing electrical field. When material is exposed to this electrical field, polar molecules gradually align. The resistance encountered during this alignment is converted to heat.
In microwave baking, the dish and the solid ingredients are relatively microwave inert and are heated at a slower rate, while the liquids, especially the moisture, are heated more rapidly than the solid ingredients. In direct contrast, in oven baking, the dish and solid structure are heated relatively quickly while the moisture is heated more slowly. Also, microwave energy has a limited depth of penetration. Microwave energy can interact with cake ingredients to cause deleterious textural changes in the finished product. Moreover, while both conventional and microwave oven baking dry mixes include flour, sugar, shortening and flavorings, each ingredient's different molecular makeup gives it unique heating properties in the microwave oven. For example, when various dry mix suspensions are heated with microwave energy, they tend to bubble-up onto the rim of the container. This characteristic of microwave cooking is particularly troublesome because a consumer may inadvertently grasp the container at the rim and experience discomfort from the hot food product.
Unfortunately the standard thermoformed plastic container design used to prepare dry mix desserts does not account for the tendency of product to bubble up onto the rim of the container.